Derby — Occurrence of Topaz near Ouro Preto, Brazil. 33 



region (and various other similar ones in Brazil) of an ancient 

 volcanic series represented by both massive and clastic types, 

 characterized by soda-bearing rocks and ranging from soda- 

 granite through augite- and nepheline-syenite to the extremely 

 basic phases rich in metallic oxides that characterize these 

 groups. In this regard the analyses by Gorceix of various 

 schists from the Boa Yista topaz mine showing comparatively 

 high soda contents are particularly significant. Perhaps also 

 the association, at least apparent, of iron and manganese ores 

 with the schists here discussed, may be taken as an indication 

 in the same direction, but a discussion of the matter must be 

 deferred to another occasion. On this hypothesis the later 

 phases of the eruptives may be supposed to have been charac- 

 terized by topaz-bearing dikes. 



Although topaz has not been recorded as occurring in rocks 

 of the types above mentioned, there is no apparent reason why 

 it should not be found, since the only element required for its 

 formation that is not normally present in them is fluorine, 

 which in the form of fluorite and fiuo-silicates is quite frequent 

 in their segregations and drusy cavities. It is worthy of note 

 that its almost constant companions in the granitic rocks in 

 which it has hitherto been found, cassiterite and tourmaline, 

 are only represented by the latter in insignificantly small pro- 

 portions. Its characteristic companion, if it can be said to 

 have one, is euclase, containing the element glucinum, charac- 

 teristic of a number of the rare minerals of the nepheline- 

 syenite pegmatites of southern Norway. It may also be noted 

 that in these last mentioned rocks cerium occurs in forms 

 different from the common one of monazite, thus presenting a 

 certain analogy with its occurrence as florencite in the topaz- 

 bearing earth of the Ouro Preto district.* 



Whether the above guess as to the original character of the 

 topaz-bearing earth proves correct or not, it is certain that the 

 occurrence of the topaz in the Ouro Preto district does not 

 differ so materially from the other known ones as has hitherto 

 been supposed. That is to say, the mineral does not occur in 

 an essentially magnesian rock nor is its matrix of presumably 

 sedimentary rather than of eruptive origin. On the other 

 hand, the rock cannot be positively identified with any in 

 which the mineral has hitherto been found. Of the known 

 modes of occurrence the only one that offers a more or less 

 remote analogy is that described by Cross in the lithophyses of 

 a rhyolite. of Colorado and Utah. As lithophyses are essen- 

 tially drusy cavities of peculiar character and as rhyolites differ 



*TLis mineral, which was first detected by Hussak in alluvial washings from 

 Tripuhy in the immediate vicinity of the topaz mines, has since been found m a 

 number of diamond residues from the Diamantina district. 



Am. Jour. Sci. — Fourth Series, Yol. XI, No. 1. — January, 1901. 

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